It is well-known to deionize water by passing water to be treated through a packed bed of ion exchange resins so that impurity ions are removed as absorbed ions on the ion exchange resin. It is well-known to regenerate the ion exchange resin having its absorbing ability lowered, by means of an acid or alkali. However, a disadvantage of such a method is the discharge of the waste liquid of the acid or alkali used for the regeneration. It is desirable, therefore, to employ a method for producing deionized water which requires no acid or alkali for regeneration. In this respect, attention has been drawn in recent years to a self-regenerating type electrodialytic deionized water producing method wherein ion exchange resins are used in combination with ion exchange membranes, commonly known as electrodeionization. The physical apparatus associated with this method is known as an electrodeionization unit.
The typical electrodeionization unit comprises alternately arranged anion exchange membranes and cation exchange membranes, thereby defining diluting compartments and concentrating compartments. Each of the diluting compartments contains an ion exchange solid composition, comprising both anion exchange and cation exchange solid compositions normally in the form of granules or beads. Water to be purified is passed through the ion exchange granules in diluting compartments.
Anionic impurities in the water are absorbed by the anion exchange solid composition and, under the influence of an applied voltage across the electrodeionization unit, migrate towards and through the anion exchange membrane to enter a first adjacent concentrating compartment for discharge in an aqueous stream flowing therethrough. Similarly, cationic impurities in the water are absorbed by the cation exchange solid composition and migrate towards and through the cation exchange membrane to enter a second adjacent concentrating compartment for discharge in an aqueous stream flowing therethrough.
Typically, the ion exchange solid composition comprises a mixture of cation exchange and anion exchange solid composition beads in a random distribution, more commonly referred to as a "mixed bed" ion exchanger. The mixed bed ion exchange material characteristically has high interfacial contact area between the anion exchange and cation exchange materials contained therein. Such contact area promotes efficient removal of ionic impurities. Further, it facilitates ionization of water molecules. By ionizing water molecules, hydrogen and hydroxyl ions are made available for regeneration of the ion exchange materials. This is particularly critical where the concentration of dissolved ionic impurities in the water to be purified is low, such as near the discharge of the diluting compartment.
Undesirably, where mixed bed ion exchange materials are used, the thickness of the diluting compartment must be necessarily thin. As thickness of the diluting compartment is increased, transport efficiency of impurity ions through the ion exchange solid composition to the membranes is reduced, thereby reducing product water quality. This is because anion exchange material is less likely to be "connected" to the anion exchange membrane by a chain of contiguous anion exchange materials (such attribute hereinafter referred to as "connectivity"). Also, cation exchange material is less likely to be connected to the cation exchange membrane by a chain of contiguous cation exchange material.
Thinner diluting compartments dictate higher manufacturing costs. Further, the effective membrane area increases for thinner diluting compartments, with a concomitant increase in material cost.
U.S. Pat. No. 4,636,296 discloses an electrodeionization unit apparatus which mitigates the above-described connectivity problem with mixed bed ion exchanges in electrodeionization unit diluting chambers. In particular, the electrodeionization unit disclosed therein comprises diluting compartments containing alternating layers of anionic exchange and cationic exchange materials. In this respect, chains of contiguous anion and cation exchange materials are provided for facilitating migration of cationic and anionic impurities respectively. However, such layering of the anion and cation exchange materials reduces interfacial contact between anion and cation exchange materials, thereby reducing removal efficiency with respect to ionic impurities and reducing the rate of ionization of water molecules at a given applied voltage. To this end, the apparatus disclosed in U.S. Pat. No. 4,636,296 relies on regenerant hydrogen and hydroxyl ions produced at the surfaces of the electrodes which, thereafter, migrate into the ion exchange solid composition. However, this approach requires multiple, costly electrodes and introduces complexity which prevents the benefits of a multipolar ionic electrochemical reactor from being realized.
PCT Application No. WO97/34696 discloses an electrodeionization unit apparatus which enhances connectivity of diluting compartment ion exchange materials, including those of the mixed bed variety. This is accomplished by using ion exchange materials wherein a pressure of from 0.1 to 20 kg/cm.sup.2 is exerted between the ion exchange materials accommodated in the diluting compartments and the cation exchange membranes and anion exchange membranes defining the diluting compartments, thereby enhancing interfacial contact between individual ion exchange materials. However, where greater internal pressures are exerted between ion exchange materials, void fractions within the diluting compartment decrease, thereby increasing resistance to water flow and reducing the amount of water capable of being treated.
Accordingly, it is desirable to have an arrangement of ion exchange material with improved connectivity and wherein the ion exchange material promotes the efficient ionization of water so that hydrogen and hydroxyl ions are available for regenerating the ion exchange material. Further, it is desirable to improve the connectivity of ion exchange materials which exert an internal pressure of 0.1 to 20 kg/cm.sup.2 between themselves.